This invention relates to the construction of the crankshaft and cylinder block of a two stroke cycle internal combustion engine to prevent the passage of charge air between the respective crankcase compartments of adjacent cylinders of the engine.
Engines operating on the conventional two stroke cycle require the incoming air charge to the cylinders of the engine to be compressed to an above atmospheric pressure in order that the air charge will flow into the engine cylinder while the exhaust gas from the previous cycle is being discharged. There are two basic modes of providing the required degree of compression of the charge gas, one being to provide a compressor (supercharger) and the other to effect compression of the charge air in the engine crankcase by the downward movement of the piston during the exhaust stroke. Engines employing the latter procedure for compressing the air charge are commonly referred to as crankcase compression two stroke cycle engines and require the crankcase to be effectively sealed in order to achieve the required degree of compression of the air charge to be generated by the movement of the piston during the exhaust stroke. Accordingly, in a multi cylinder two stroke cycle engine operating on the crankcase compression system requires an individually sealed crankcase compartment to be provided for each cylinder of the engine.
Another characteristic of two stroke cycle engines operating on the crankcase compression system is that the crankcase can not be used as a reservoir for lubricating oil, and oil from a pressure circuitry lubrication system can not be permitted to be directly delivered into the crankcase. Accordingly, the crankshaft and connecting rod bearings can not normally be of the conventional plain metal type, and are usually of the anti friction type such as ball or roller bearings, which can effectively operate with minimal lubrication. As such bearings do not form a seal between adjacent crankcase compartments as is provided by a plain metal bearing, provision must be made to obtain an effective seal between the cylinder block and crankshaft journals, where the crankshaft passes through the dividing wall between adjacent crankcase compartments of a multi two stroke cycle cylinder engine, to prevent the passage of charge air from one to the other through the anti-fraction bearing.
The need to provide both a bearing and a seal within the wall separating adjacent crankcase compartments normally requires the centre distance between adjacent cylinders to be increased so that the dividing wall is of sufficient width to accommodate the axial length of the ball or roller bearing together with the axial length of an appropriate seal structure. This results in an increase in the centre distance of adjacent cylinders which is contrary to the requirement of minimising the axial length of multi cylinder engines to reduce the total weight thereof, to reduce the torsional vibration in the crankshaft, and to reduce the space requirement of the engine compartment of a vehicle, thus contributing to overall reduction in weight of the vehicle, and the drag co-efficient of the vehicle body.
The above problem has led to a proposed constructions wherein the centre distance between the cylinder can be reduced and the required bearing and seal support is provided by increasing the width of the wall below the level of the cylinders. However, this construction gives rise to potential problems in manufacture as the portion of the wall having the increased width can laterally project into a location below the bore of the cylinders on one or both sides of the wall. This lateral projection of the wall to below the cylinder can interfere with the machining of the bore of the cylinders and the assembling of the piston into the cylinder. Both of these operations are preferably performed from the crankcase end of the cylinders for accuracy and convenience consideration.